Lightweight vehicle structures are now being developed in which larger components are being made from Carbon Fibre Reinforced Plastic (CFRP) and other composite materials such as glass-filled thermoplastics. There is a need to provide robust, load-bearing threaded connections between these composite components and also to other parts of the vehicle which may be made of other materials, typically metal.
Self-tapping screws may be used to provide load bearing interconnections, but it is generally not desirable to drive such screws directly into such composite materials due to their low strength and the delamination damage caused by the penetration of screw thread crests into the fibre reinforced material. Such delamination then undesirably reduces the strength and may damage the appearance, of the composite material. Similarly, conventional press fit threaded inserts or studs with engaging splines and knurls are undesirable as the penetration of the composite by the sharp edges of the splines or knurls cause delamination of the CFRP material.
Alternatively a flanged, threaded metal nut or stud may be moulded within the composite material, during layering of the composite material in manufacture, which in turn serves as a connection with a mating screw, bolt or nut. These fasteners provide releasable connection of composite components to other components. However such moulded-in inserts require a time consuming, high-skill-level assembly process and may require complex and expensive-to-make mould tools to accommodate the metal insert. Additionally ingress of molten plastic or resin into the thread form of the fastener may be difficult to avoid.
Other fastening options to provide a threaded metal fastener on a composite structure include studs and nuts bonded onto the panel surface using adhesive or other mechanical fastening means such as that described in U.S. Pat. Nos. 6,773,780-A, 4,778,702-A or 5,704,747-A which describe a surface mounted, adhesive bonded threaded stud and nut respectively U.S. Pat. No. 5,704,747-A in particular requires a separate flaring step to flare the edge of a liner sleeve to hold the fastener in place at least during adhesive curing. In addition to delayed assembly times, these components are complex to assemble. Such bonded fasteners will easily move within the panel until the adhesive has cured, thus making handling difficult or impossible until the adhesive cures adequately. This then increases manufacturing costs through delay and increased storage requirements for stock during manufacture. Furthermore, the strength of the completed fixing is limited by the bond strength of the adhesive and the delamination strength of the panel itself. Additionally, adhesives are well known for having relatively low peel resistance thus limiting bending resistance in a purely adhesive-bonded configuration.
US 2009/0169324-A discloses a fastener designed to couple thin-walled structures together. The fastener comprises a male portion with an external screwthread for mating with a corresponding internal screwthread of a female portion in order to clamp the walled structure therebetween. However, it is said that shear load across a threaded fastener, such as the male portion, results in stress risers across the threads. This can cause the fastener to fail. So this disclosure teaches use of a sleeve between the male and female portions to serve as a shear bush in order to absorb shear load and obviate thread failure. The only coupling together in this fastener is between the male and female portions, not the sleeve and that coupling is by way of screwthread engagement.
GB 2,477,550-A discloses a fixing insert to provide a threaded bore in a plate. The insert is formed in two halves, one male and one female which are coupled together across the plate via screwthread engagement. The force with which the two fastener halves apply compression to the sandwiched plate depends on how much torque is applied to their screwthread engagement. There is no disclosed means for controlling the axial separation of the two fastener halves.